This invention relates to devices for the treatment of heart disease and particularly to stents. More specifically, the invention relates to a method and system for removably securing the stent to a catheter for delivery through a body lumen and subsequent implantation. The invention has particular usefulness in securing stents coated with heat sensitive material.
A variety of non-surgical interventional procedures have been developed over the years for opening stenosed or occluded blood vessels caused by the build up of plaque or other substances on the walls of the blood vessel. Such procedures usually involve the percutaneous introduction of the interventional device into the lumen of the artery, usually through a catheter. One widely known and medically accepted procedure is balloon angioplasty, in which an inflatable balloon is introduced within the stenosed region of the blood vessel to dilate the occluded vessel. The balloon catheter is initially inserted into the patient""s arterial system and is advanced and manipulated into the area of stenosis. The balloon is inflated to compress the plaque and press the vessel wall radially outward to increase the diameter of the blood vessel.
In another widely practiced procedure, the stenosis can be treated by placing an expandable interventional instrument such as an expandable stent into the stenosed region to expand and hold open the segment of blood vessel or other arterial lumen. Stents are particularly useful in the treatment or repair of blood vessels after percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or by atherectomy. Stents are usually delivered to the target site in a compressed, or crimped, condition, and then are deployed into an expanded condition to support the vessel and help maintain it in an open position.
One method and system developed for delivering stents within a patient""s body lumen involves crimping a stent about an expandable member, such as an angioplasty balloon on the distal end of a catheter, advancing the catheter through the patient""s vascular system until the stent is in the desired location within a blood vessel, and then inflating the expandable member on the catheter to expand the stent within the blood vessel. The expandable member is then deflated and the catheter withdrawn, leaving the expanded stent within the blood vessel, holding open the passageway.
Retaining the position of the stent in the proper location on the expandable member while advancing the catheter through the body lumen has sometimes proven difficult. If the stent is dislodged from or moved on the expandable member, the system will not correctly deliver the stent into the body lumen.
Different methods have been attempted to maintain the position of the stent on the expandable member. One such method involves a protective sheath surrounding the catheter and stent assembly. The sheath is retracted prior to the inflation of the expandable member. The use of the sheath, however, increases the profile of the catheter assembly and decreases the flexibility of the system. Another method involves removing the friction reducing coating on the expandable member in the location of the stent, thus allowing the catheter assembly""s pre-coated surface to hold the stent in frictional contact.
Another method of securing the stent to the balloon catheter is described in U.S. Pat. No. 5,836,965 to Jendersee et al. The method, while not completely clear, appears to include the steps of positioning a stent onto a balloon within an encapsulating sheath, pressurizing the balloon while elevating the temperature of the balloon, depressurizing the balloon, and then removing the encapsulating sheath. In the final configuration the balloon extends through or around a portion of the stent.
A significant improvement to the Jendersee method is described in U.S. application Ser. No. 09/391,859, filed Sep. 8, 1999, to Foreman, Limon, et al. and assigned to Advanced Cardiovascular Systems, Inc., the assignee of the present invention. That application is incorporated by reference in its entirety. It describes how local deformations can be formed in an expandable member, such as a balloon, by forcing an expansion fluid into the interior of the expandable member while the stent is crimped on it. In this manner the expandable member partially inflates into the gaps in the stent. To restrain the expandable member from expanding during deformation of the expandable member, an inelastic sheath can be placed about the stent. The inelastic sheath allows the internal pressures in the expandable member to exceed the levels which would otherwise expand the expandable stent. A further improvement is to heat the expandable member while it is being deformed, to a sufficient temperature and for a sufficient duration to permanently deform the expandable member to form bulges which fill the gaps (i.e.,voids) between the struts of the stent. Advanced Cardiovascular Systems, Inc. (ACS) of Santa Clara, Calif., the assignee of this invention, calls this method the GRIP process, in which a high temperature nozzle traverses the entire length of the stent several times while the sheathed balloon remains pressurized. Nozzle temperatures can approach 200xc2x0 F.
The advent of coated stents has rendered these prior art methods of stent attachment ineffective. Coated stents, particularly those coated with drugs, are often temperature sensitive, and cannot withstand the heat required in the prior art methods. In the past, one solution would have been to simply sheath the stent, but that is no longer acceptable. The increasing trend to direct stentingxe2x80x94placing the stent without first opening the stenosis by balloon angioplastyxe2x80x94requires lower profile stents while insuring that the stents remain secured to the balloon until placed at the lesion and expanded. With some coatings, sensitive to temperatures of approximately 50xc2x0 C. (122xc2x0 F.), like actinomycin-D, no satisfactory method has been developed to use heat in securing crimped stents to balloons.
What has been needed and unavailable is a means of mounting and maintaining a coated stent at a desired location on a stent delivery system while using a heat-seated balloon, but without increasing the overall profile of the catheter assembly and thus compromising flexibility. The present invention satisfies this need.
This invention is directed to an improved method and system for securing coated stents to delivery systems. Securing the stent is accomplished by tightly crimping the stent onto the balloon portion of a catheter. An expansion restraint is placed over the stent and the balloon is pressurized. The balloon at the end of the stent conforms to the stent""s geometry. The stent is kept cool by a stent temperature controller while a portion of the catheter balloon extending beyond the edge of the stent is heated by a heat source, such as hot air. The heat permanently seats the balloon at the stent""s ends. In one embodiment, an insulating material is disposed between the heat source and the stent temperature controller to further protect the stent. In this manner, the stent is kept cool enough so that any temperature sensitive coatings are not deleteriously affected while heat is applied to the balloon. The basic system for accomplishing this method includes the heat source, the insulator, and the stent temperature controller.
Additional steps can be added to the method to provide for treating both ends of the balloon and for quenching the balloon once it has been adequately heated. The method is preferably accomplished by using the embodiment of a system for securing the stent that is disclosed in detail below. The system includes a hollow chiller block that is fed cool air from a chiller. The chiller block keeps the stent cool. The block is separated from heat nozzles by a rubber insulating disk. The disk insulates the stent from the heat that the nozzles apply to the end of the balloon protruding out from the stent and beyond the disk.
The present invention results in a simplified method of inserting the stent into the body lumen, because no sheath is required to secure the stent. The catheter assembly is inserted into the body lumen without further steps being taken to secure the stent. The absence of a sheath makes the assembly more flexible than it would be with a sheath. The expandable member is inflated at the desired location, expanding and implanting the stent within the body lumen. When the expandable member is then deflated, the stent is released and the remainder of the catheter assembly may be withdrawn, leaving the stent implanted within the body lumen.